The present invention relates to a method for adjusting a cryogenic refrigeration apparatus and to a corresponding apparatus.
The invention relates more particularly to a method for adjusting a cryogenic refrigeration apparatus comprising several refrigerators/liquefiers arranged in parallel to cool one and the same application, each refrigerator/liquefier comprising a working circuit for a working gas equipped with at least one valve for controlling the flow of working gas, the refrigerator/liquefiers in parallel using a working gas of the same kind such as pure gaseous helium, each refrigerators/liquefier comprising a working gas compression station, a cold box intended to cool a flow of working gas leaving the compression station to a cryogenic temperature at least close to its liquefaction temperature, said flows of working gas cooled by each of the respective cold boxes of the refrigerators/liquefiers being mixed and then placed in a heat exchange relationship with the application in order to give up frigories thereto, the cold working gas having exchanged heat with the application then being divided into several return flows distributed respectively through the respective compression stations.
The invention relates to what is referred to as “large-scale” refrigeration apparatuses employing several refrigerators/liquefiers in parallel in order to cool one and the same user application
A “refrigerator/liquefier” denotes a device which subjects a working gas (for example helium) to a thermodynamic cycle of work (compression/expansion) that brings the working fluid to a cryogenic temperature (for example a few degrees K in the case of helium) and where appropriate liquefies this working gas.
One nonlimiting example of such an apparatus is described in application no. FR2980564A1.
The refrigeration cycles (which generate cold) are said to be “closed” at the level of each refrigerator. What that means to say is that the flow of working gas that enters the cold box of a refrigerator/liquefier reemerges for the most part from this same cold box. By contrast, the flow of working gas is said to be “open” at the level of the application that is to be cooled, which means to say that the gas from the various refrigerators/liquefiers is mixed therein. The flow of working gas supplied by the refrigerators/liquefiers is therefore pooled for cooling the application then returned separately to each refrigerator by a distribution system.
Adjustment of the refrigerators of such an apparatus generally involves manually positioning the control valves of the working circuit (from and to the application that is to be cooled).
Suitable adjustment becomes more difficult when the apparatus comprises a great many interfaces and when the thermal loads that need to be cooled vary over time. This is because static adjustment of the valves may be unsuitable if the flow rate and/or pressure of the system vary.
The fluctuating thermal loads of the application indeed generate fluctuations in the flow rate through the compressors.
If this is not corrected, certain refrigerators/liquefiers will recuperate more working gas and cold than others. Thus, certain refrigerators/liquefiers may diverge from their nominal operating point. Certain components of these refrigerators/liquefiers may therefore be used at their limit (compressors, turbines, etc.) whereas the other refrigerators/liquefiers will be underutilized. The overall cold power of the apparatus and the efficiency thereof will therefore be reduced.
Providing systems for control and adjusting the independent flows for each refrigerator/liquefier may lead to a system which overall is unstable in which the loads and flow rates will be distributed inconsistently between the refrigerators/liquefiers. In addition, the specific features of helium (a density that varies greatly as a function of temperature) lead to a phenomenon in which the imbalances between the refrigerators are amplified.
The distribution of helium flow rates between the refrigerators is performed generally via a common helium feed pressure and the resistance (pressure drop) of the circuit returning to the source of pressure (compressors).
When one refrigerator/liquefier receives in relative terms more cold gas coming from the application, the mean temperature of the return circuit drops and the pressure drop of the circuit is therefore reduced. Specifically, the density of the gas may change more rapidly than the speed of the gas through the circuit. This drop in pressure drop in a circuit leads to a relative increase in the flow rate of cold gas accepted into the circuit concerned and therefore leads to divergence within the apparatus.
It is an object of the present invention to alleviate all or some of the disadvantages mentioned hereinabove of the prior art.